This invention relates to ion generating apparatus comprising a cross field discharge device utilizing a PIG (Penning Ion Gauge) type discharge.
Recently, in surface wording (ion plating, ion implantation, etching, etc.) and surface analysis of semiconductors and metals or in a field of nuclear fusion and nuclear physics, various kinds of ion generating apparatus have been widely used. In these apparatus are generally included duoplasmatron-type, duopigatron-type and the like type ion generating apparatus in which a plasma is created and ions generated are extracted. However, these apparatus commonly possess a disadvantage of an adverse gas efficiency, which results in the entrainment of a large amount of neutral gaseous molecules in an ion beam extracted from the ion generating apparatus and an electric breakdown is easily caused at an ion accelerating portion. For this reason, it is difficult to obtain high ion beam energy and the plasma is created about the ion beam extracted. Moreover, electrons are accelerated at the ion acclerating portion towards an ion generating source in opposite direction of the ion flow and collide with an electrode. In addition, electrons increase in their numbers on the way towards the electrode by ionization of the gas molecules flown from the ion source, thus rapidly increasing the temperature of the electrode, which results in breakage of the electrode limiting its long time use. A further disadvantage of the prior type ion generating apparatus (ion source) resides in the short life time of a hot cathode provided for the apparatus to supply electrons.
An improved PIG type ion generating apparatus provided with a cold cathode and having a good gas efficiency has been proposed for obviating the disadvantages described above.
FIG. 1 shows a typical ion generating apparatus of this type, which comprises a vacuum vessel or vacuum envelope 5, a cylindrical anode 1 located in the vessel 5, a pair of cathodes 3 and 4 located in the vessel 5 at portions near both end openings of the anode 1 so as to cover the inner tubular, usually cylindrical, hollow portion 2 of the anode 1 with small gaps, lead wires 6 and 7 electrically connected to the anode 1 and cathodes 3 and 4 respectively, and a magnetic field generating device 8 surrounding the vacuum vessel 5 for applying a magnetic axially of the anode. The annular cathode 3 is disposed on the side of extracting ions (leftside as viewed in FIG. 1) of the anode 1 and is provided with a central through hole 9 communicating with the hollow portion 2 of the anode 1. The other cathode 4 is disposed on the rightside of the anode and the both cathodes 3 and 4 are electrically connected so as to have a common potential. The anode 1 is connected to a d.c. power source 27 and the cathodes are grounded through a current measuring device 25 which measures cathode current Ik.
The opened end of the vacuum vessel 5 is connected to an evacuating device provided with exhausting means provided for a surface analyzer, for example, not shown, and the interiors of the surface analyzer and the vacuum vessel 5 are preliminarily exhausted to maintain a desired degree of vacuum in the analyzer. The evacuating device is well known itself by those skilled in the art, for example, in "Technical Information" from Institute of Plasma Physics, Nagoya University, Japan, October, 1979. In the use of an ion generating apparatus described above, a desired gas, such as He gas, is admitted from a gas source through the evacuating device into the vacuum vessel 5 to establish a gas discharge in the inner hollow portion 2 of the anode 1 thereby generating ions. The generated ions are ejected into the surface analyzer through the hole 9 to heat analyze the surface of a material to be dealt with. Although the operating condition of the ion genrating apparatus can be selected in accordance with the use thereof, one example will be shown as follows. The density of gas charged in vacuum vessel 5 is 1.times.10.sup.17 m.sup.-3 ; the radius of hollow portion 2 is 7.5 mm; the interelectrode voltage is 5 KV; and the intensity (strength) of magnetic field is 0.15 T (tesla), and ions are ejected in an arrowed direction.
According to the prior art ion generating apparatus of the type described above in conjunction with FIG. 1, ions can be extracted through a hole provided for a cathode by utilizing such a feature as that the ions created by the PIG type discharge collectively collide with the surface of the cathode. Thus, an ion generating apparatus provided with cold cathodes and having a high gas efficiency can be produced. However, as stated in (1) J. C. Helmer and R. L. Jensen's paper entitled "Electrical Characteristics of a Penning Discharge", Proc, IRE, 49(61), 1920 and (2) W. Knaner's paper entitled "Mechanism of the Penning Discharge at Low Pressures", J. Appl. Phys. 33(62) 2093, disadvantages of the apparatus of this type reside in that kinetic energy of the ion beam is distributed in a wide range and it is very difficult to construct the beam line so as to improve the focusing and the parallelism of the ion beam.
In another point of view, the ion generating apparatus according to this invention includes heavy ion generating apparatus in which a heavy ion generation material is disposed on one of the cathodes of the apparatus.
Well known ion generating apparatus includes electron bombardment type, PIG type, and duoplasmatron type heavy ion generating apparatus, in which ions of a material in solid state at a room temperature can be generated.
However, the electron bombardment type apparatus requires a vapour generating furnace for generating vapour of a desired material and the use of such furnace often contaminates the interior of the ion generating apparatus and makes worse the operability thereof. The duoplasmatron type apparatus requires large electric power for forming plasma and a hot cathode or a hollow cathode, which makes worse the operability of the apparatus. The PIG type apparatus utilizing sputtering phenomenon has the advantage that many kinds of heavy ions are generated without using a high temperature vapour generating furnace, but it also requires a large electric power and it is difficult to suppress the temperature rise due to the use of the large electric power.
Although there have been porposed other type ion generating apparatus utilizing sputtering and having cathodes provided with through holes, which have simple construction and consume less electric power, they are not suitable for actual use because of considerable small current of ions taken out in comparison with the other type apparatus described hereinbefore.
In still another point of view, the ion generating apparatus include apparatus each having cathodes covering the end openings of an anode and provided with through holes, respectively. However, when these apparatus are used under the same conditions as those described with respect to the apparatus shown in FIG. 1, discharge state, e.g. discharge current and space voltage are unstable and good operability of the apparatus cannot be obtained.